The present invention relates generally to honeycomb cell materials or structures and a method of manufacture of such material.
Honeycomb cell structures are commonly used as seals in jet aircraft and other industrial applications. The honeycomb structure must normally be bent into a diameter to form a jet engine seal since regions in jet engines to be sealed are normally round, such as the outer periphery of a turbine. The honeycomb core or cell material is typically formed of stainless steel or other high temperature metal alloys, and is brazed to a backing strip or mounting ring. Brazing of honeycomb cell structures to a backing strip is normally carried out using brazing powder or brazing tape. However, such a technique is subject to errors and inconsistencies due to the fact that there is no consistency in the amount of brazing powder used. Excessive amounts will cause some wicking into the cell walls and thicker joint areas in some regions, and can even create filled cells. If too little brazing powder is used, an insufficient joint may be provided in some areas of the assembly.
Honeycomb cell structures have been proposed in the past which have a brazing foil or strip incorporated into the honeycomb cells. For example, U.S. Pat. No. 4,491,265 of Ittner describes manufacture of a honeycomb cell array with embedded strips of corrugated brazing foil. In order to form a honeycomb cell array, strips of metal are formed into zig-zag shapes and then welded together at their nodes to form a series of generally hexagonal cells. Ittner""s method involves placing a corrugated brazing foil strip between adjacent metal strips and welding it in place at the nodes. The brazing foil strips within the honeycomb will then function to braze the honeycomb assembly to panels or other structures. However, this structure is very rigid, due to the I-beam effect of the brazefoil between the hex cells. The resultant cell structure cannot be bent into a diameter, and it is therefore completely unsuitable for forming engine seals and the like.
It is an object of the present invention to provide a new and improved honeycomb cell assembly incorporating brazing foil strips within the cell structure.
It is a further object of the present invention to provide a new and improved method of manufacturing a honeycomb cell structure with embedded brazefoil.
According to one aspect of the present invention, a honeycomb cell structure is provided, which comprises a plurality of first strips each formed into a generally zig-zag configuration and placed side by side to form a plurality of rows of cells in a generally honeycomb configuration with nodes between adjacent cells in each row, and a plurality of second strips of brazing foil, each second strip being positioned between adjacent first strips and the first and second strips being secured together at each node such that the foil strip extends across each cell of the honeycomb, and each second, brazing foil strip having opposite side edges and a predetermined width between the side edges, and having a series of spaced slits extending transversely from at least one side edge across at least a distance equal to greater than half of the width of the second strip, at least some of the slits being located within respective cells of the honeycomb.
In an exemplary embodiment of the invention, at least one slit is located in each cell of the honeycomb structure or at least every other cell in each row of the honeycomb. The brazing foil has alternating slits extending from opposite side edges in an exemplary embodiment, with each slit extending across a distance between ⅝ and xc2xe of the foil width. In an alternative embodiment, the foil strip is split completely across its width in each cell, to form two completely separate portions in each cell. In each case, the slits or splits in the foil strip will make the resultant honeycomb structure more flexible, so that it can be bent to adopt a desired diameter or conform to the intricate design of a jet turbine engine. At the same time, the honeycomb structure has all the advantages of having a built-in brazing foil so that it can be directly brazed to a desired backing strip, avoiding the need to use brazing powder or a separate brazing tape.
Another advantage of incorporating brazing foil with slits or splits into the honeycomb cell structure, as compared with a conventional brazefoil honeycomb which has no slits, is that there will be less stress on the brazing foil strips, producing a more accurate arid uniform braze. Conventional brazefoil honeycomb tends to distort, flex or bow by a significant amount when heated during brazing, due to the stress on the brazing foil. This invention provides slits or splits in the brazing foil strip, in every cell or at least every other cell, removing the stress on the foil which may otherwise cause distortion of the honeycomb structure during brazing.
According to another aspect of the present invention, a method of making a brazefoil honeycomb structure is provided, which comprises the steps of:
forming a plurality of first strips into a generally zig-zag configuration such that each side face of each strip forms one side of a respective row of honeycomb cells separated by nodes;
positioning a pair of strips face-to-face at a first spacing;
placing a second strip of brazing foil between the pair of first strips;
bringing the first and second strips together such that the second strip is sandwiched between the pair of first strips and extends across the cells formed between the strips;
securing the first and second strips together at the nodes; and
repeating the preceding four steps until a honeycomb structure of predetermined dimensions is formed;
each second strip having a plurality of spaced slits along its length, each slit extending transversely across the strip for a distance equal to at least greater than the width of the strip.
In a first embodiment of the invention, spaced slits are formed in the second strips prior to assembly in the honeycomb structure, with the slits extending alternately from opposite side edges of the strip and being at a predetermined spacing such that at least one slit is located in at least every second cell in each row. In an exemplary embodiment, one slit is located in every cell in the honeycomb.
In an alternative method, slits are formed in the second strips after assembly into the honeycomb structure, by means of a laser cutting machine or the like. In this alternative, the second strip may be cut completely across its width in each cell, so that two completely separate brazing foil sections are provided in each cell. This increases flexibility and improves uniformity of the braze.